THE SEGMENTATION OF THE OVUM. 121 



(2) The distribution of food-yolk in the protoplasm of the 

 ovum exercises an important influence on the segmentation. 



The rapidity with which any part of an ovum segments varies 

 ceteris paribus with the relative amount of protoplasm it contains; 

 and the size of the segments formed varies inversely to the 

 relative amount of protoplasm. When the proportion of pro- 

 toplasm in any part of an ovum becomes extremely small, 

 segmentation does not occur in that part. 



Ova with food-yolk may be divided into two great groups 

 according to the eventual arrangement of the food-yolk in the 

 protoplasm. In one of these, the food-yolk when present is 

 concentrated at the vegetative pole of the ovum. In the other 

 group it is concentrated at the centre of the ovum. Ova belong- 

 ing to the former group are known as telolecithal ova, those to 

 the latter as centrolecithal. 



In each group more than one type may be distinguished. In 

 the first group these types are (i) unequal segmentation, (2) 

 partial segmentation. The features of these three types have 

 been already so fully explained that I need not repeat them here. 



In the second group there are three distinct types, (i) equal 

 segmentation, (2) unequal segmentation. These two being ex- 

 ternally similar to the similarly named types in the first group. 

 (3) Superficial segmentation. This is unlike anything which is 

 present in the first group, and is characterized by the appearance 

 of a superficial layer of cells round a central yolk mass. These 

 cells may either appear simultaneously or successively, and their 

 nuclei are derived from the segmentation within the ovum of the 

 first segmentation nucleus. 



The types of ova in relation to the characters of the segmen- 

 tation may be tabulated in the following way : 



Segmentation. 



(i) alecithal ) 



regular 



ova l 



(2) telolecithal j (a) unequal 



ova ) (b) partial 



(3) centra- } ^ re ular ( with segments united in 



lecithal 1 central yolk mass) 



ova \ W unequal 



(c) superficial. 



